Processing-property-microstructure relationships in TiAl-based alloys
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TRODUCTION
A large research program is under way at the Interdisciplinary Research Centre (IRC) in Materials for High Performance Applications aimed at developing optimized compositions and processing routes for alloys based on TiAl. Some recent results, obtained through this program, are presented in this article. II. EXPERIMENTAL One-kilogram experimental buttons of alloys have been produced using a single plasma torch that melts precompacted feedstock onto a water-cooled copper hearth. These buttons are usually inverted and melted up to 5 times in an attempt to improve their homogeneity. A preliminary assessment of the forgeability and the properties of these alloys is carried out on the button material, and ingots of the most promising alloys are produced as 1-m-long and 100mm- or 150-mm-diameter ingots using a twin torch plasma melter for a more comprehensive assessment of processibility and properties. Forging has been carried out using the IRC 500 tonne isothermal forge capable of operating at strain rates down to approximately 1023 s21 up to about 1200 7C in either vacuum or argon. A limited amount of hot extrusion has been carried out in industry. The typical oxygen level both for the buttons and for the ingots is between 600 and 750 ppm. Powder has been produced from sections of the plasmamelted ingots using a bottom pouring cold-wall induction
D. HU, A. GODFREY, and P.A. BLENKINSOP, Research Fellows, and M.H. LORETTO, Head of the Department, are with the Interdisciplinary Research Centre in Materials for High Performance Applications, The University of Birmingham, Edgbaston, United Kingdom B15 2TT. This article is based on a presentation made in the symposium ‘‘Fundamentals of Gamma Titanium Aluminides,’’ presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
furnace to melt the alloy. When the charge is molten, the nozzle, which in the current design is made of graphite, is heated so that the liquid metal flows out and is atomized using argon. Under normal operating conditions the pickup of carbon is negligible for the Ti aluminides, but not surprisingly is far worse for other Ti alloys. For the powder used in this work the typical C content was 0.02 wt pct and the oxygen between 650 ppm for the size fraction above 150 mm and 1000 ppm for particles below 45 mm. Hotisostatic pressing (‘‘hipping’’) has been used to produce samples suitable for assessing the mechanical properties of these powder-route materials. Long-term heat treatments of Ti-47Al-2Ta-1Cr-1Mn0.2Si, with and without 1 pct B, and Ti-48Al-2Nb-2Cr-1B have been carried out in air at 700 7C. Optical microscopy has been used to assess the microstructure of the alloys and to examine fracture surfaces. A JEOL* 840 analytical scan*JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.
ning electron microscope has been used to examine fracture surfaces and to assess the phase distribution in th
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